To print every second element, print inside the condition as shown below:

for(int i=0;i<10;i++) {//esnure the brace here       
   if (i%2 == 0) {
       System.out.println(i);//prints every second element
   }
}

You missed the curly braces of for loop.

public class Generalizations {
    public static void main(String[] args) {


        for(int i=0;i<10;i++){
            System.out.println(i);
            if (i%2==0){
                System.out.println("hello");
            }
        }
        }
    }

use this

public class Generalizations {
public static void main(String[] args) {


    for(int i=0;i<10;i++){
        System.out.println(i);
        if (i%2==0){
            System.out.println("hello");
        }
    }
}